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I am interested in finding the summer average (either climatalogical mean or for a specific year) relative humidity and temperature for a set of globally distributed latitude and longitude coordinates.

What I need are mean values for June, July, and August.

I would prefer to use actual observed values, but a reanalysis (such as NARR or CRU-NCEP would suffice). Is there a web-based interface from which I can obtain this information?

The closest that I have come to this

  • Wolfram Alpha, but they only provide rainfall and temperature, not relative humidity

  • worldclim.org looks promising, but requires me to download all of the data before using another program to extract information; it also does not have relative humidity.

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2 Answers 2

up vote 7 down vote accepted

Weather Underground users can now access daily weather conditions dating back six years in most cities around the world. Historical weather information such as maximum and minimum temperatures, precipitation, humidity and winds are available for use by researchers, travelers, event planners and weather buffs free of charge. The databank was recently expanded to include hourly weather figures beginning with the 1999 archives in most markets.

http://www.wunderground.com/about/pr/news.asp?date=19990412

Use the API to access this data

Historical Request Example (San Fransisco Airport) on 18th October 2010.

http://api.wunderground.com/api/YOUR_API_KEY/history_20101018/q/SFO.json

Share Response
HTTP/1.1 200 OK

X-CreationTime:
    0.262
Date:
    Wed, 17 Oct 2012 13:27:47 GMT
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    25571
Expires:
    Fri, 01 Jan 1990 00:00:00 GMT
Set-Cookie:
    DT=##################v4; path=/; expires=Fri, 01-Jan-2020 00:00:00 GMT; domain=.wunderground.com
Content-Type:
    application/json; charset=UTF-8
Connection:
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Server:
    Apache/1.3.33 (Unix) PHP/4.4.0
Pragma:
    no-cache
Cache-Control:
    no-cache, must-revalidate, no-cache="Set-Cookie", private


{
  "response": {
    "version": "0.1",
    "termsofService": "http://www.wunderground.com/weather/api/d/terms.html",
    "features": {
      "history": 1
    }
  },
  "history": {
    "date": {
      "pretty": "October 18, 2010",
      "year": "2010",
      "mon": "10",
      "mday": "18",
      "hour": "12",
      "min": "00",
      "tzname": "America/Los_Angeles"
    },
    "utcdate": {
      "pretty": "October 18, 2010",
      "year": "2010",
      "mon": "10",
      "mday": "18",
      "hour": "19",
      "min": "00",
      "tzname": "UTC"
    },
    "observations": [
      {
        "date": {
          "pretty": "12:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "00",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "7:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "07",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "15.0",
        "tempi": "59.0",
        "dewptm": "8.3",
        "dewpti": "46.9",
        "hum": "64",
        "wspdm": "0.0",
        "wspdi": "0.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "North",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.9",
        "pressurei": "30.06",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 180756Z 00000KT 10SM FEW017 BKN110 15/08 A3006 RMK AO2 SLP179 T01500083 401560144"
      },
      {
        "date": {
          "pretty": "1:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "01",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "8:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "08",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "14.4",
        "tempi": "57.9",
        "dewptm": "8.9",
        "dewpti": "48.0",
        "hum": "70",
        "wspdm": "9.3",
        "wspdi": "5.8",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "180",
        "wdire": "South",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.5",
        "pressurei": "30.05",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Scattered Clouds",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 180856Z 18005KT 10SM FEW017 SCT120 14/09 A3005 RMK AO2 SLP175 T01440089 58009"
      },
      {
        "date": {
          "pretty": "2:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "02",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "9:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "09",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "13.9",
        "tempi": "57.0",
        "dewptm": "8.3",
        "dewpti": "46.9",
        "hum": "69",
        "wspdm": "0.0",
        "wspdi": "0.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "North",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.9",
        "pressurei": "30.06",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Scattered Clouds",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 180956Z 00000KT 10SM R28R/3500VP6000FT FEW017 SCT120 14/08 A3006 RMK AO2 SLP179 T01390083"
      },
      {
        "date": {
          "pretty": "3:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "03",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "10:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "10",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "12.8",
        "tempi": "55.0",
        "dewptm": "8.3",
        "dewpti": "46.9",
        "hum": "74",
        "wspdm": "9.3",
        "wspdi": "5.8",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "220",
        "wdire": "SW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.3",
        "pressurei": "30.04",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181056Z 22005KT 10SM BKN017 BKN120 13/08 A3004 RMK AO2 SLP173 T01280083 $"
      },
      {
        "date": {
          "pretty": "4:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "04",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "11:56 AM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "11",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "13.9",
        "tempi": "57.0",
        "dewptm": "8.9",
        "dewpti": "48.0",
        "hum": "72",
        "wspdm": "0.0",
        "wspdi": "0.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "North",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.3",
        "pressurei": "30.04",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Overcast",
        "icon": "cloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181156Z 00000KT 10SM BKN015 OVC017 14/09 A3004 RMK AO2 SLP173 70005 T01390089 10150 20122 58002"
      },
      {
        "date": {
          "pretty": "5:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "05",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "12:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "12",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "15.0",
        "tempi": "59.0",
        "dewptm": "9.4",
        "dewpti": "48.9",
        "hum": "69",
        "wspdm": "11.1",
        "wspdi": "6.9",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "150",
        "wdire": "SSE",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.0",
        "pressurei": "30.04",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181256Z 15006KT 10SM BKN014 BKN017 15/09 A3003 RMK AO2 SLP170 T01500094"
      },
      {
        "date": {
          "pretty": "6:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "06",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "1:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "13",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "15.0",
        "tempi": "59.0",
        "dewptm": "9.4",
        "dewpti": "48.9",
        "hum": "69",
        "wspdm": "7.4",
        "wspdi": "4.6",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "160",
        "wdire": "SSE",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.6",
        "pressurei": "30.05",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Overcast",
        "icon": "cloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181356Z 16004KT 10SM OVC014 15/09 A3005 RMK AO2 SLP176 T01500094"
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        "date": {
          "pretty": "7:56 AM PDT on October 18, 2010",
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          "mday": "18",
          "hour": "07",
          "min": "56",
          "tzname": "America/Los_Angeles"
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        "utcdate": {
          "pretty": "2:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "14",
          "min": "56",
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        "tempm": "15.0",
        "tempi": "59.0",
        "dewptm": "9.4",
        "dewpti": "48.9",
        "hum": "69",
        "wspdm": "7.4",
        "wspdi": "4.6",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "290",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.8",
        "pressurei": "30.06",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Overcast",
        "icon": "cloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181456Z 29004KT 10SM OVC014 15/09 A3006 RMK AO2 SLP178 T01500094 53005"
      },
      {
        "date": {
          "pretty": "8:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "08",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "3:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "15",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "15.6",
        "tempi": "60.1",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "69",
        "wspdm": "7.4",
        "wspdi": "4.6",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "180",
        "wdire": "South",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1018.1",
        "pressurei": "30.07",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Overcast",
        "icon": "cloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181556Z 18004KT 10SM OVC014 16/10 A3007 RMK AO2 SLP181 T01560100"
      },
      {
        "date": {
          "pretty": "9:12 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "09",
          "min": "12",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "4:12 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "16",
          "min": "12",
          "tzname": "UTC"
        },
        "tempm": "16.0",
        "tempi": "60.8",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "68",
        "wspdm": "5.6",
        "wspdi": "3.5",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "Variable",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.8",
        "pressurei": "30.06",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "SPECI KSFO 181612Z VRB03KT 10SM SCT014 BKN160 16/10 A3006 RMK AO2"
      },
      {
        "date": {
          "pretty": "9:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "09",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "4:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "16",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "17.2",
        "tempi": "63.0",
        "dewptm": "10.6",
        "dewpti": "51.1",
        "hum": "65",
        "wspdm": "0.0",
        "wspdi": "0.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "North",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1018.2",
        "pressurei": "30.07",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181656Z 00000KT 10SM SCT014 BKN160 17/11 A3007 RMK AO2 SLP182 T01720106"
      },
      {
        "date": {
          "pretty": "10:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "10",
          "min": "56",
          "tzname": "America/Los_Angeles"
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        "utcdate": {
          "pretty": "5:56 PM GMT on October 18, 2010",
          "year": "2010",
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        "wspdm": "0.0",
        "wspdi": "0.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "0",
        "wdire": "North",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1018.0",
        "pressurei": "30.06",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181756Z 00000KT 10SM FEW014 BKN160 17/11 A3006 RMK AO2 SLP180 T01720106 10178 20139 50002"
      },
      {
        "date": {
          "pretty": "11:56 AM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "11",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "6:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "18",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.6",
        "dewpti": "51.1",
        "hum": "61",
        "wspdm": "9.3",
        "wspdi": "5.8",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "330",
        "wdire": "NNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.8",
        "pressurei": "30.06",
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        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Mostly Cloudy",
        "icon": "mostlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181856Z 33005KT 10SM FEW010 SCT160 BKN180 18/11 A3006 RMK AO2 SLP178 T01830106"
      },
      {
        "date": {
          "pretty": "12:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "12",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "7:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "19",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "300",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1017.1",
        "pressurei": "30.04",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 181956Z 30013KT 10SM FEW010 18/10 A3004 RMK AO2 SLP171 T01830100"
      },
      {
        "date": {
          "pretty": "1:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "13",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "8:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "20",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "37.0",
        "wgusti": "23.0",
        "wdird": "300",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1016.7",
        "pressurei": "30.03",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182056Z 30013G20KT 10SM R28R/2200VP6000FT FEW010 18/10 A3003 RMK AO2 SLP167 T01830100 58013 $"
      },
      {
        "date": {
          "pretty": "2:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "14",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "9:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "21",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.9",
        "tempi": "66.0",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "56",
        "wspdm": "20.4",
        "wspdi": "12.7",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "290",
        "wdire": "WNW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1016.1",
        "pressurei": "30.01",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182156Z 29011KT 10SM FEW010 19/10 A3001 RMK AO2 SLP161 T01890100 $"
      },
      {
        "date": {
          "pretty": "3:56 PM PDT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "15",
          "min": "56",
          "tzname": "America/Los_Angeles"
        },
        "utcdate": {
          "pretty": "10:56 PM GMT on October 18, 2010",
          "year": "2010",
          "mon": "10",
          "mday": "18",
          "hour": "22",
          "min": "56",
          "tzname": "UTC"
        },
        "tempm": "18.3",
        "tempi": "64.9",
        "dewptm": "10.0",
        "dewpti": "50.0",
        "hum": "58",
        "wspdm": "24.1",
        "wspdi": "15.0",
        "wgustm": "-9999.0",
        "wgusti": "-9999.0",
        "wdird": "310",
        "wdire": "NW",
        "vism": "16.1",
        "visi": "10.0",
        "pressurem": "1015.7",
        "pressurei": "30.00",
        "windchillm": "-999",
        "windchilli": "-999",
        "heatindexm": "-9999",
        "heatindexi": "-9999",
        "precipm": "-9999.00",
        "precipi": "-9999.00",
        "conds": "Partly Cloudy",
        "icon": "partlycloudy",
        "fog": "0",
        "rain": "0",
        "snow": "0",
        "hail": "0",
        "thunder": "0",
        "tornado": "0",
        "metar": "METAR KSFO 182256Z 31013KT 10SM FEW010 18/10 A3000 RMK AO2 SLP157 

truncated for GIS Stack Exchange.

http://www.wunderground.com/weather/api/ (can be setup in 5 minutes)

For Lat/Lngs you need to add the GeoLookUp request:

http://api.wunderground.com/api/YOUR_API_KEY/geolookup/q/37.776289,-122.395234.json
http://api.wunderground.com/api/YOUR_API_KEY/geolookup/q/CA/San_Francisco.json
share|improve this answer
    
That is a phenomenal resource! And there is a command-line interface too github.com/sramsay/wu! Could you give an example of how I could query, e.g., daily average temperature and relative humidity for a given lat/lon? –  Abe Oct 16 '12 at 22:18
    
Have created a json historical request/response example (San Fransisco Airport) on 18th October 2010. See updated answer (full response was truncate as GIS SE has a limitation on the amount of characters you can have in an answer) –  Mapperz Oct 17 '12 at 13:37
    
This is working very well ... but the closest that I have gotten to what I need is the "planner" feature to get climatealogical means for a date range, e.g. api.wunderground.com/api/MY_API_KEY/planner_06010831/q/…. It returns Dew Point instead of Relative Humidity; but there is a sufficient way to estimate RH. –  Abe Oct 17 '12 at 16:24
    
argggh ... it seems that the conversion from dewpoint to RH is non-trivial (as discussed by Lawrence 2005) –  Abe Oct 17 '12 at 16:50
    
I asked how to solve t_d = f(rh) at physics.SE –  Abe Oct 17 '12 at 17:21

Microsoft Research provides an interactive web interface FetchClimate and an associated API interface for use with C, C#, and other .NET frameworks.

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